Interference-fit terminal pins are widely used in printed wiring board backplanes to interconnect not only electrical components on either side of the backplane but also to make connections to circuitry carried on the backplane. An interference-fit pin must be designed so that sufficient retention forces are generated when it is inserted into a compatible plated-through hole of the backplane. It is equally important to minimize deformation of the hole plating and surrounding epoxy so that a reliable connection can be achieved if and when a replacement pin must be inserted. Such deformation may occur when pins of uniform cross section are fitted into backplane plated-through holes having varying plating thicknesses due to permissible plating tolerances. These tolerances have been increased, and thereby the plating cost reduced, by employing pins having an insertion portion which complies with varying hole sizes.
One terminal pin having a compliant insertion portion of an S-shaped cross section is disclosed in U.S. Pat. No. 3,444,617 of A. A. Stricker et al. issued May 20, 1969. In this and in other known arrangements, after the pin has been fully inserted in the backplane hole, its insertion portion flexes to adjust to the hole diameter, the spring action of the pin insertion portion maintaining positive electrical contact. However, the high retention force requirement imposes a definite limit on the compliance of the insertion portion. In the past, compliant terminal pins have proved largely satisfactory when the diameter of the plated-through holes has been held within fairly close tolerances. Where attempts are made to further increase the tolerances to achieve an additional reduction in plating costs, existing pins have proved inadequate because the transition section between the pin proper and the flexible compliant portion has either been so rigid that it deforms the plating of an oversized plated-through hole and thereby degrades the electrical connection achieved upon full insertion or so compliant that it does not sufficiently "condition" an undersized plated-through hole to receive the fully developed cross section yet to come. Accordingly, it is an object of the present invention to provide a compliant contact pin wherein the stiffness of the transition section encountered by a backplane plated-through hole varies precisely with the hole diameter.